(1) Field of the Invention
The present invention relates to a digital echo repeater/beacon system for use with marine vessels. The system of the present invention has been found to have particular utility as part of test platforms used during sea tests of marine systems.
(2) Description of the Prior Art
Acoustic arrays such as acoustic echo repeaters for underwater communication and retransmission are known in the art. They are frequently employed to test acoustic systems and as target simulators One such underwater acoustic repeater array is illustrated in U.S. Pat. No. 3,421,137 to Rathbun et al.
The Rathbun et al. repeater array has the ability to be used as both an echo repeater and as a target simulator. It consists of three equispaced transducers supported concentrically between the apexes of a pair of truncated equilateral metallic pyramids. The outer surface of the pyramids are coated with an acoustic reflecting material and the active faces of the transducers are directed toward the coated faces. A housing having an amplifier therein is connected between two such arrays. The housing includes electrical devices for connecting in parallel one set of the transducers to the amplifier input and the output thereof to the other three transducers. The acoustic signal which is received by one set of transducers is amplified and re-radiated by the other transducer set at a higher level with a minimum of coupling therebetween.
Other acoustic target simulators are shown in U.S. Pat. Nos. 3,555,165 to Ettenhofer et al. and 3,721,949 to Hubbard et al. The Ettenhoffer et al. apparatus is capable of simulating acoustic echoes of sonar pulses from a desired selected target configuration. The apparatus includes a hydrophone whose output signal is fed to a signal processor for modifying a received sonar pulse and generating an electrical echo analogue signal which is applied to a projector for transmission. The signal processor includes a modulator for down modulating the carrier frequency of the received pulse and delay devices for delaying the pulse by varying degrees equivalent to the travel times for the pulse between successive major echo producing discontinuities in the target whose echo is being simulated to form a series of pulses. The pulses are controlled in amplitude in accordance with the appropriate target strengths for the succession of discontinuities and are fed to fill-in and summing circuits for adding white noise of desired envelope characteristics in the intervals between successive pulses to form the echo analogue signal. A second modulator shifts the frequency band of the echo analogue signal upwardly and introduces a selected Doppler shift thereinto prior to its application to the projector for acoustic transmission. Feedback elimination circuitry delays and shifts the phase of the echo analogue signal and applies the resulting signal to a summer for eliminating the effect of acoustic feedback.
Prior art test systems used in the field often fail to provide adequate data for analysis of test performance. In some systems, returns can only be obtained every few minutes when operating ranges are in excess of 100 KYards. When obtained these returns are often not detectable due to low signal level and infrequent and unpredictable arrival times.
Prior art systems also suffer the deficiencies of being costly to produce and having little flexibility in terms of their capability.